There remains a need for the ability to detect chemicals from a stand-off position across numerous industries and applications including, but not limited to: forensic crime scene analysis; border, checkpoint, portal and facility protection; cargo and facility screening and inspection; surveillance; and even stockpile and production monitoring. Specifically, applications requiring or benefitting from chemical detection include, for example: screening of people, vehicles, cargo as they attempt to cross borders, enter checkpoints and enter public or other high traffic facilities including airports, train stations, sports and/or concert venues, office buildings, shopping centers and the like; detection of residue or pre-cursor or constituent materials from explosives, poisons, narcotics or other hazardous materials; tracking people and things through surveillance.
However, current systems do not provide the sensitivity, specificity, and low false-alarm rates that are needed to enable effective use in a cluttered, real-world environment. Current techniques for detecting chemicals in the field range from collecting samples and transporting them back to a laboratory for analysis, to small point sensors that alert to the presence of a single chemical or chemical class, to passive or active optical sensors that can search the ground for chemical targets from an airborne platform. Each different chemical detection method has both strengths and limitations.
Laboratory analysis techniques such as Nuclear Magnetic Resonance spectroscopy (NMR), mass spectrometry, Fourier Transform Infrared (FTIR) spectroscopy, and various forms of chromatography provide precise chemical identification from very small quantities of sample material. But there is a time lag of hours to days for a sample to be collected and transported to the laboratory and collecting enough samples to comprehensively analyze large areas for trace surface residues is cost and time prohibitive. Field-portable versions of several of these techniques do exist, which reduce analysis time to minutes, but to test for surface residue samples must still be collected by wiping or swabbing the surface(s) of interest. Also, the sensitivity and specificity of field-portable instruments is significantly lower than the performance capability of their laboratory counterparts.
Optical spectroscopy based standoff techniques are the most viable approach for rapid, high area coverage chemical detection of trace residues on surfaces. But while a number of existing standoff optical spectroscopy techniques such as fluorescence spectroscopy, differential absorption light detection and ranging (DIAL), Raman spectroscopy, and laser induced breakdown spectroscopy (LIBS) offer either high sensitivity or high specificity, none can simultaneously provide the needed performance metrics in both categories. Many optical standoff techniques also have additional drawbacks, such as eye safety concerns.
Existing infrared hyperspectral imaging field techniques can identify chemicals in limited cluttered environment cases but cannot simultaneously achieve the required sensitivity and selectivity levels needed for most applications. In contrast, the basic physics of active IR absorbance and reflectance spectroscopy at the needed sensitivity and selectivity levels is well developed for laboratory environments. FTIR spectrometers can rapidly and efficiently analyze gas, liquid, and solid phase samples with 0.1 cm−1 or better spectral resolution and standard sensitivities typically 10-100 times higher than many application requirements for trace chemical detection. However, translating the performance of state of the art IR spectrometers to a portable, active standoff capability is currently limited by a number of factors related both to the spectrometer itself, and the post-acquisition spectroscopic analysis needed to extract chemical identification information from a multi-component high resolution absorption or reflectance spectrum.
With respect to the spectrometer, current standoff active IR techniques are limited by sources and detectors. There have historically been two choices for active illumination: broadband incoherent light sources that require large collimating optics and have limited range, or narrow-band coherent sources that limit spectral coverage and therefore sensitivity. In addition, dispersive spectrometer-based detection, used with many current passive and actively illuminated systems, intrinsically trades signal-to-noise (SNR), resolution, and scan speed, because the collected photons must be dispersed and separated into individual resolution elements prior to detection.
For example, open-path FTIR relies on efficient retro-reflectors that preserve the transmitted energy and use large collimating optics to project the thermal energy of the infrared source. Even with these advantages, large apertures are still required. To project five orders of magnitude more energy on to a target than a standard open-path system, it is necessary to use lasers. Possible configurations include tunable lasers that scan the spectrum over time, dense frequency combs that generate a multitude of narrow lines, and super-continuum (SC) lasers that produce spectrally continuous output. Tunable laser sources in the mid-IR have been around for many years. Frequency-agile CO2 lasers use gratings to switch among the multitude of lines generated by the source. QCLs represent the new generation in mid-IR laser sources. They offer better efficiency than CO2 and broad tunability. QCLs can be made to cover all parts of the mid-IR and beyond. A tunable laser-based system scans the spectrum by quickly switching wavelengths over its “tuning range,” so a sensor that relies on it would be very simple, only requiring a broadband detector because the spectroscopy is built into the source. The disadvantage of tunable source-based systems is that by only illuminating one line at a time, the overall efficiency is very low compared with a source that can illuminate many wavelengths at once.
Dense frequency combs can be generated with QCLs. Assuming that the power per line is the same as with a tunable source, combs have an efficiency advantage of a factor equal to the number of lines over the tunable source, at the expense of complexity in the receiver, which must now perform the spectroscopic function (or by modulating the source—either way, complexity is unavoidable). However, they are still discrete by nature and therefore only provide information on the spectral lines for which they are designed. Unlike frequency combs, SC sources are continuous and thus their output energy interacts with all of the absorption lines from the materials it encounters. The disadvantage of SC sources has been the requirement for pulsed lasers, unlike CO2 or QCL lasers, which are continuous-wave, but with the availability of fast detectors and electronics, the present embodiments capture the individual pulses and reject the background energy while reducing the detector noise via shorter integration times.
Accordingly, there remains a need in the art for a system for standoff detection and identification of trace chemical residues on surfaces using active infrared spectroscopy at a distance. A preferred system would feature portability and real-time results with high chemical sensitivity and specificity across a broad range of target classes and effective operation in a real-world environment accounting for issues such as gas phase and surface-adsorbed clutter, varying substrates, temperature, humidity, indoor/outdoor background light.